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Most people think of Alzheimer's as an old person's disease, but doctors say it can actually start formulating in your brain as early as your 30s. While that might be a scary thought, there are a few lifestyle changes you can make now that experts believe could impact your risk of developing the disease.

"One out of three cases of Alzheimer's may be preventable if that person does everything right," explained Dr. Richard Isaacson, director of the Alzheimer's Prevention Clinic at Weill Cornell & New York-Presbyterian Medical Center in New York City.

"Exercise can protect against Alzheimer's because it not only increases blood flow to the brain, but it loosens up that amyloid plaque, the bad sticky stuff that gets caught up and gunked up in the brain of a person with Alzheimer's disease," noted Isaacson.

Any exercise helps, but experts recommend getting at least three hours of rigorous activity a week. Ideally, that would be two cardio workouts and one strength-training session.

Avoid sugar and processed foods, and you might want to switch to a Mediterranean diet. The brain shrinks as you age, but a study published last year in the journal Neurology found people in their mid-70s who consumed a Mediterranean diet (more fruits, veggies, olive oil, and less meat and cheese) lost less brain mass than people who ate a diet more typical of their country, Scotland. A bigger brain later in life is beneficial and could protect from diseases like Alzheimer's.

Keep your brain challenged, TODAY correspondent Maria Shriver noted. Shriver recommended the website BrainHQ, which features many brain exercises that have been linked to dramatically lower rates of dementia in seniors.